The long terminal repeats (LTRs) of the human immunodeficiency virus (HIV) the Rous sarcoma virus (RSV) and the copia Drosophila retrotransposon were compared in their capacity to direct expression of the bacterial cat (chloramphenicol acetyltransferase) gene in human, murine, and Drosophila cell lines. The results indicate that HIV and RSV LTR expression is post transcriptionally repressed in the Drosophila cells while copia LTR expression is post-transcriptionally repressed in the human and murine cells.